1. Field of Invention
The present invention relates to an electronic timepiece that receives satellite signals and obtains date and time information from GPS satellites or other positioning information satellites.
2. Description of Related Art
The Global Positioning System (GPS), which is a system for determining one's location, uses GPS satellites that orbit the Earth on known orbits, and enables devices to determine the current location of a GPS receiver by receiving signals from the GPS satellites.
To capture satellite signals transmitted from the GPS satellites, threshold values are assigned to correlation and signal strength values, and satellite signals with a signal level exceeding the threshold value are received to determine if subsequent periodic processes and navigation data decoding processes can be executed. This enables receiving high quality radio signals with low noise, and enables improving positioning precision and correcting the time and date accurately. See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2003-149315.
When the GPS receiver taught in JP-A-2003-149315 receives a satellite signal from a satellite at a high elevation angle, the signal level of the signal received from that satellite is stored in memory, and when the number of data values stored in memory exceeds a specific number, the data average (average movement) is calculated. This GPS receiver uses a method of eliminating satellite signals from satellites at a low elevation angle and eliminating multipath interference by referencing a data table correlating optimum threshold values to received signal levels, and setting an optimum threshold value for the average movement.
This GPS receiver acquires and stores almanac data in local memory. Based on the almanac data stored in memory, the GPS receiver can identify the GPS satellites located at a high elevation angle and receive satellite signals from the identified GPS satellites, which is particularly useful for so-called warm-start reception processes.
However, this GPS receiver sets a threshold value for receiving satellite signals from GPS satellites located at a high elevation angle based on the almanac data regardless of the reception environment, and receives satellite signals with signal strength equal to or exceeding the threshold value. As a result, when the reception environment is poor, such as when the GPS receiver is located indoors, the GPS receiver taught in JP-A-2003-149315 cannot capture satellite signals from GPS satellite located at a high elevation angle and therefore cannot receive signals.
A further problem in small electronic timepieces such as wristwatches is that providing storage capacity sufficient to store the almanac data is difficult. In addition, acquiring the almanac data requires a lengthy reception process, and because the power supply is limited in such small electronic timepieces, such lengthy reception processes are not desirable because of their power consumption.
Such small electronic timepieces therefore use a correlation process to identify the transmission source of the satellite signal by applying a pseudonoise code (PN code) to determine from which unknown positioning information satellite the received signal was transmitted. If the signal strength of the satellite signal identified by this correlation process is greater than or equal to a specified signal reception threshold value, the satellite signal is captured and received. In other words, such small electronic timepieces require a so-called cold-start reception process. A problem with this configuration is that if a high signal reception threshold value appropriate to satellites at a high elevation angle is set in this cold-start satellite signal reception process and the reception environment is poor, the time from the start of the signal correlation process until the satellite signal is captured is long, and power consumption therefore increases.